1. Field of the Invention
The present invention relates to a proximity sensor, and in particular to a proximity sensor for use in semiconductor lithographic applications.
2. Related Art
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked upon. In some situations, such as semiconductor lithography, that distance must be measured with an accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of lithography systems. In the lithography context, in addition to the needs to be non-intrusive and to accurately detect very small distances, the proximity sensor cannot introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the quality of the material surface or product being worked upon.
Different types of proximity sensors are available to measure very small distances. Examples of such proximity sensors include capacitance gauges and optical gauges. However, these proximity sensors have serious shortcomings when used in lithographic projection systems because the physical properties of materials deposited on wafers may impact the accuracy of these sensors. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. More generally, optical and capacitive methods are prone to errors due to significant interactions with layers beneath photoresist coatings. Another class of problem occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance gauges and optical gauges may provide spurious results, and are therefore not optimal.
A typical gas gauge pressure sensor contains a reference nozzle and one or more measurement nozzles to emit a gas flow onto reference and measurement surfaces. Measurements are made of the back pressure differences within the sensors to determine the distance between the measurement nozzle and the measurement surface. Such a gas gauge pressure sensor is not vulnerable to concentrations of electric charges or to the electrical, optical or other physical properties of a wafer surface. A gas gauge pressure sensor detects only the top physical layer, and thereby yields a superior result. Accordingly, these types of gauges are ideal for topographic measurement of a material surface, such as that used to establish focus prior to lithographic exposure.
Speed of measurement is a critical performance driver in current semiconductor manufacturing processes. Specifically, increased bandwidth of proximity sensors is necessary to support current semiconductor manufacturing throughput practice. Traditional gas gauge proximity sensors use outward gas flows from which pressure changes are used to derive the desired proximity measurement. However, gas pressure fluctuations do not propagate rapidly in a direction against the gas flow, as is the case in a traditional gas gauge proximity sensor. The outward gas flows at the point of measurement affect the speed of propagation of the pressure changes. This resistance, and thereby the response time constant, increase when the proximity sensor uses lower pressures, or equivalently higher gas flow rates. In fact, operating conditions can result in the gas flow rate at the measurement nozzles approaching sonic conditions. Thus, the propagation of the pressure changes resulting from changes in the substrate surface in the vicinity of these measurement nozzles are significantly slowed by such a rapid outward gas flow. Accordingly, the response time and therefore the bandwidth potential of such proximity sensors suffer dramatically as a result of this physical phenomenon.